Preparation Of PDMS-based Pervaporation Membranes And Study On Their Separation Performance For Bioalcohols

Biofuel is an efficient substitute for fossil fuels,which is mainly represented by biobutanol and bioethanol.They can be obtained through the lignocellulose fermentation.However,the key issue is the extremely low solvent concentration in the fermentation broth(e.g.1-butanol concentration?13 g L-1),limiting the industrial production.It results in the energy-intensive downstream process,of which the energy consumption takes about 1/2-2/3 of the whole production process.Thus,developing the high-efficiency separation technology of butanol is the key to reduce energy consumption and cost,and enhance the market competitiveness of butanol.Pervaporation technology has more advantages such as energy saving,environmental sustainability and less harmful effects on microorganisms,which is encouraged to integrate with the fermentation process to achieve the in situ solvents separation.As the core of separation,membrane directly determines the energy consumption of operation unit.Thus,developing a high-efficient membrane preparation technology and enhancing membrane separation performance have important practical significance.Polydimethylsiloxane(PDMS)is the most widely used membrane materials.This study aims to develop an ultrafast and continuous preparation technology of PDMS membrane;further,propose a particle-driven enhancement strategy for pervaporation performance.The main contents are shown below.In view of the problem that the conventional preparation process of PDMS membrane based on the thermal crosslinking is time-consuming(usually?24 h),uncontrollable and hard to realize the continuous preparation,the preparation technology of PDMS pervaporation membrane with high efficiency was build.FT-IR,1H NMR and viscosity tests confirm the successful synthesis of MA-PDMS.UV rheology and real-time infrared spectroscopy results show that MA-PDMS can be fully cured within 30 s,and the polymerization rate is increased by three orders of magnitude than that of thermal crosslinking.The crosslinking density,Tg and pervaporation experiments determined the compact structure and good performance of UV-crosslinked PDMS membrane.Importantly,the free-solvent,continuous and controllable preparation technology has been successfully scaled up on an industrial scale,which greatly improves the productivity.To improve the pervaporation performance of MA-PDMS membrane,incorporating fillers into polymer matrix to enhance the transport property of penetrate molecules was used.However,filler aggregation easily results in the decline of separation performance.Based on the ultrafast UV curing,a freezing effect towards fillers to improve the dispersibility was proposed.SEM show that when the silicalite-1 loading was up to 45 wt%,it still had a uniform distribution in MA-PDMS layer.The 1-butanol separation factor and 1-butanol permeability of 45 wt%MMM were improved by 49%and 14%,respectively,when it was used to separate 1.5 wt%1-butanol/water solution at 55?.However,the physical incorporation method easily causes the defects between polymer and fillers.The covalent bridging between two phases was used to improve their compatibility and avoid forming defects.FT-IR,XPS and N2 adsorption-desorption tests confirm the successful synthesis of MA-silicalite-1,where it has the UV-polymerized property.Water contact angle tests show that the value of silicalite-1 was increased by 53° after modification,displaying the high hydrophobicity.UV rheology shows the defects between MA-silicalite-1 and MA-PDMS were eliminated by the covalent bridging.Further,in view of the problem that the currently reported network cavity size of PDMS membranes is smaller than the kinetic size of ethanol,which increases the transfer resistance,the strategy using MA-silicalite-1 as nanospacer to adjust the membrane structure was proposed.Free volume property exhibits that network cavity size was increased by 0.069 nm after doping MA-silicalite-1,breaking through the kinetic radius of ethanol.It confirms the increased interstitial cavities in the cross-linked sites and the loose chain packing.When 50 wt%MA-silicalite-l/MA-PDMS MMM was used to separate 5 wt%ethanol/water solution at 60?,the ethanol flux was up to 922 g m-2 h-1 with 13.4 of separation factor.The separation factor was increased by 61.4%in comparation with membrane without no fillers.It shows the feasibility of the above strategy,and provides an effective approach for designing the high-performance membrane structure.In addition to silicalite-1,ZIF-8 has been widely used in PDMS based MMMs due to its permanent porosity and high hydrophobicity.Nevertheless,the degradation of ZIF-8 framework occurs in acid environment,it is hard to be directly used in the real ABE fermentation broth system.Thus,ZNC with the stable carbon framework was prepared by carbonizing ZIF-8,and further used to replace ZIF-8 in MMM.After ZIF-8 and ZNC were treated by"butyric acid+acetic acid" model aqueous solutions,N2 adsorption-desorption and SEM tests confirm that the structure stability was observably improved by carbonization.Meanwhile,the pore structure was improved,where total pore volume and surface area were increased by 0.55 cm3 g-1 and 105 m2 g-1,respectively.Therefore,when used to separate the model ABE solution,ZNC/PDMS MMM overcome the trade-off effect.Importantly,100 h of continuous operation feeding ABE realistic fermentation broth,membrane performance was stable with membrane flux of 1870 g m-2 h-1 and separation factor of 20.Further,mixed matrix system involves the environment pollution caused by organic solvents and the defects caused by filler aggregation and poor filler/polymer compatibility,making the large-scale preparation difficult.This study used the water particles instead of the conventional porous particles to enhance the transfer structure,realizing a green preparation process of membrane.Real-time infrared spectroscopy demonstrates the water-based MA-PDMS membrane can be fully cured in seconds,while the double bonds conversion was between 76%and 83%.Free volume property shows that the ortho-positronium intensity was increased from 30.74%to 33.89%after introducing the water particles into MA-PDMS layer.It proves that water particles were frozen in membrane,and further increased the pore porosity.Meanwhile,the free volume cavities and the fractional free volume were both increased,confirming the increased interstitial cavities in the cross-linked sites and the loose chain packing.When separating 1 wt%1-butanol/water solution at 60?,the total flux of water-based MA-PDMS membrane was increased by 10%-23%compared with the water-free MA-PDMS membrane.It shows the feasibility of the above strategy,and provides an effective approach for designing the membrane structure with high flux.